1. Technical Field
The present invention relates to a method and apparatus for frequency modulation in general, and in particular to a method and apparatus for demodulating frequency modulation signals. Still more particularly, the present invention relates to a method and apparatus for demodulating digital frequency modulation signals in communication systems.
2. Description of the Prior Art
Frequency modulation (FM) is utilized in many types of communication systems, such as FM radio broadcast, television, analog cellular telephony, and a variety of remote sensing applications in which measured analog signals are transmitted from a remote site to a central site for processing. Traditionally, FM signals are demodulated using analog processing techniques. However, in recent years, analog signal processing has been largely replaced by digital signal processing (DSP) in order to exploit the many advantages of digital processing techniques.
Because the information in FM signals is encoded within a time varying phase angle of a sinusoidal signal, most of the prior art digital FM demodulation algorithms utilize inverse trigonometric functions in general, and the inverse tangent (also called arc tangent) in particular, for extracting the encoded information. Generally speaking, there are two principal methods that are commonly utilized to compute the inverse trigonometric functions for FM demodulation: (i) consulting a lookup table that maps each measured FM signal value with a corresponding angle value, and (ii) approximating the inverse trigonometric functions using a polynomial. One disadvantage of the lookup table method is that high-performance FM demodulation requires a relatively large lookup table, which implies a large amount of memory is needed. Another disadvantage of the lookup table method is that additional processing such as signal scaling and point interpolation between table entries are typically required. On the other hand, there are also disadvantages associated with the polynomial approximation method; one disadvantage being a large amount of processing time is required to compute the approximating polynomial. Thus, in order to achieve high-performance FM demodulation with the existing methods, a designer must choose between large memory requirement (for the lookup table method) and long processing time (for the polynomial approximation method). In addition, prior art digital methods typically require a higher sampling rate than is necessary to accurately digitize the signal. This higher sampling rate is needed to avoid degradations due to noise induced wrapping of positive digital frequencies into negative digital frequencies or vice versa.
Consequently, it would be desirable to provide an improved method for demodulating digital FM signals, with no increase in memory requirement, processing time, and sampling rate.